In previous work (Ng et al, 2012) we have applied the KIM method with offline segmentation to calculate the 3D prostate trajectory after treatment. In the present study we are utilising online marker segmentation to enable live trajectory creation during treatment delivery with a processing time less than 1 second. With the circa real-time trajectories we are able to gate the delivery so to maximise dose to the tumour, or track with Multi-Leaf Collimators (MLC) to follow the prostate motion. The latter involves complex interaction with the beam delivery system and is not part of this study. The treatment can be gated based on a pre-set tolerance. Using our earlier data we modelled several gating criteria 3mm/5s, 3mm/10s and 3mm/15s based on excursions along individual axes and also the radial excursion. Across the 10 patients, a tolerance of 3mm/5s was shown to be efficient, introducing only 24 gating events (from 268 treatments) and safe, with the smallest time for excursion (5s).